But today via SHH! come word that at least right now, the prospect of him writing the script or pretty much nil due to his association with Marvel Comics:
"Well, sadly, I'm a Marvel guy and we were surprised to find out that WB couldn't hire me for a DC property. They were incredibly nice and superbly apologetic about it, but when they discussed the matter seriously DC explained just how associated I am with Marvel Comics at the moment and it's against company policy to hire the competition. It's absolutely nothing personal. I spoke to some friends at DC and they explained this has happened with a couple of big Marvel writers in the last couple of years and I absolutely respect that."
Millar is very heavily associated with Marvel, being the architect of their "Ultimate" (and alternative) universe of superhero comics, which takes more of a "real world" approach to the genre. He's also the guy who brought us the very divisive, political and extremely successful "Civil War" crossover event in the Marvel Comics world, which caused a schizm in the folks-who-wear-tights community over mandatory government registration and employment for those with super-powers. A significant side effect of that series is that in the comic book world, Tony Stark aka Iron Man is now pretty much a reviled character outside of his loyal fanbase.
Anyway, in regards to the Superman story, you can head on over to SuperHeroHype! for the full quote, which is quite lengthy and has more detail regarding the situation.